.. SPDX-License-Identifier: GPL-2.0 ==========================Userspace debugging advice========================== This document provides a brief overview of common tools to debug the LinuxKernel from userspace.For debugging advice aimed at driver developers go :doc:`here</process/debugging/driver_development_debugging_guide>`.For general debugging advice, see :doc:`general advice document</process/debugging/index>`. .. contents::    :depth: 3 The following sections show you the available tools. Dynamic debug------------- Mechanism to filter what ends up in the kernel log by dis-/en-abling logmessages. Prerequisite: ``CONFIG_DYNAMIC_DEBUG`` Dynamic debug is only able to target: - pr_debug()- dev_dbg()- print_hex_dump_debug()- print_hex_dump_bytes() Therefore the usability of this tool is, as of now, quite limited as there isno uniform rule for adding debug prints to the codebase, resulting in a varietyof ways these prints are implemented. Also, note that most debug statements are implemented as a variation ofdprintk(), which have to be activated via a parameter in respective module,dynamic debug is unable to do that step for you. Here is one example, that enables all available pr_debug()'s within the file::   $ alias ddcmd='echo $* > /proc/dynamic_debug/control'  $ ddcmd '-p; file v4l2-h264.c +p'  $ grep =p /proc/dynamic_debug/control   drivers/media/v4l2-core/v4l2-h264.c:372 [v4l2_h264]print_ref_list_b =p   "ref_pic_list_b%u (cur_poc %u%c) %s"   drivers/media/v4l2-core/v4l2-h264.c:333 [v4l2_h264]print_ref_list_p =p   "ref_pic_list_p (cur_poc %u%c) %s\n" **When should you use this over Ftrace ?** - When the code contains one of the valid print statements (see above) or when  you have added multiple pr_debug() statements during development- When timing is not an issue, meaning if multiple pr_debug() statements in  the code won't cause delays- When you care more about receiving specific log messages than tracing the  pattern of how a function is called For the full documentation see :doc:`/admin-guide/dynamic-debug-howto` Ftrace------ Prerequisite: ``CONFIG_DYNAMIC_FTRACE`` This tool uses the tracefs file system for the control files and output files.That file system will be mounted as a ``tracing`` directory, which can be foundin either ``/sys/kernel/`` or ``/sys/debug/kernel/``. Some of the most important operations for debugging are: - You can perform a function trace by adding a function name to the  ``set_ftrace_filter`` file (which accepts any function name found within the  ``available_filter_functions`` file) or you can specifically disable certain  functions by adding their names to the ``set_ftrace_notrace`` file (more info  at: :ref:`trace/ftrace:dynamic ftrace`).- In order to find out where calls originate from you can activate the  ``func_stack_trace`` option under ``options/func_stack_trace``.- Tracing the children of a function call and showing the return values are  possible by adding the desired function in the ``set_graph_function`` file  (requires config ``FUNCTION_GRAPH_RETVAL``); more info at  :ref:`trace/ftrace:dynamic ftrace with the function graph tracer`. For the full Ftrace documentation see :doc:`/trace/ftrace` Or you could also trace for specific events by :ref:`using event tracing<trace/events:2. using event tracing>`, which can be defined as described here::ref:`Creating a custom Ftrace tracepoint<process/debugging/driver_development_debugging_guide:ftrace>`. For the full Ftrace event tracing documentation see :doc:`/trace/events` .. _read_ftrace_log: Reading the ftrace log~~~~~~~~~~~~~~~~~~~~~~ The ``trace`` file can be read just like any other file (``cat``, ``tail``,``head``, ``vim``, etc.), the size of the file is limited by the``buffer_size_kb`` (``echo 1000 > buffer_size_kb``). The:ref:`trace/ftrace:trace_pipe` will behave similarly to the ``trace`` file, butwhenever you read from the file the content is consumed. Kernelshark~~~~~~~~~~~ A GUI interface to visualize the traces as a graph and list view from theoutput of the `trace-cmd<https://git.kernel.org/pub/scm/utils/trace-cmd/trace-cmd.git/>`__ application. For the full documentation see `<https://kernelshark.org/Documentation.html>`__ Perf & alternatives------------------- The tools mentioned above provide ways to inspect kernel code, results,variable values, etc. Sometimes you have to find out first where to look andfor those cases, a box of performance tracking tools can help you to frame theissue. Why should you do a performance analysis?~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ A performance analysis is a good first step when among other reasons: - you cannot define the issue- you do not know where it occurs- the running system should not be interrupted or it is a remote system, where  you cannot install a new module/kernel How to do a simple analysis with linux tools?~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ For the start of a performance analysis, you can start with the usual toolslike: - ``top`` / ``htop`` / ``atop`` (*get an overview of the system load, see  spikes on specific processes*)- ``mpstat -P ALL`` (*look at the load distribution among CPUs*)- ``iostat -x`` (*observe input and output devices utilization and performance*)- ``vmstat`` (*overview of memory usage on the system*)- ``pidstat`` (*similar to* ``vmstat`` *but per process, to dial it down to the  target*)- ``strace -tp $PID`` (*once you know the process, you can figure out how it  communicates with the Kernel*) These should help to narrow down the areas to look at sufficiently. Diving deeper with perf~~~~~~~~~~~~~~~~~~~~~~~ The **perf** tool provides a series of metrics and events to further dial downon issues. Prerequisite: build or install perf on your system Gather statistics data for finding all files starting with ``gcc`` in ``/usr``::   # perf stat -d find /usr -name 'gcc*' | wc -l    Performance counter stats for 'find /usr -name gcc*':      1277.81 msec    task-clock             #    0.997 CPUs utilized     9               context-switches       #    7.043 /sec     1               cpu-migrations         #    0.783 /sec     704             page-faults            #  550.943 /sec     766548897       cycles                 #    0.600 GHz                         (97.15%)     798285467       instructions           #    1.04  insn per cycle              (97.15%)     57582731        branches               #   45.064 M/sec                       (2.85%)     3842573         branch-misses          #    6.67% of all branches             (97.15%)     281616097       L1-dcache-loads        #  220.390 M/sec                       (97.15%)     4220975         L1-dcache-load-misses  #    1.50% of all L1-dcache accesses   (97.15%)     <not supported> LLC-loads     <not supported> LLC-load-misses    1.281746009 seconds time elapsed    0.508796000 seconds user   0.773209000 seconds sys    52 The availability of events and metrics depends on the system you are running. For the full documentation see`<https://perf.wiki.kernel.org/index.php/Main_Page>`__ Perfetto~~~~~~~~ A set of tools to measure and analyze how well applications and systems perform.You can use it to: * identify bottlenecks* optimize code* make software run faster and more efficiently. **What is the difference between perfetto and perf?** * perf is tool as part of and specialized for the Linux Kernel and has CLI user  interface.* perfetto cross-platform performance analysis stack, has extended  functionality into userspace and provides a WEB user interface. For the full documentation see `<https://perfetto.dev/docs/>`__ Kernel panic analysis tools---------------------------   To capture the crash dump please use ``Kdump`` & ``Kexec``. Below you can find  some advice for analysing the data.   For the full documentation see the :doc:`/admin-guide/kdump/kdump`   In order to find the corresponding line in the code you can use `faddr2line  <https://elixir.bootlin.com/linux/v6.11.6/source/scripts/faddr2line>`__; note  that you need to enable ``CONFIG_DEBUG_INFO`` for that to work.   An alternative to using ``faddr2line`` is the use of ``objdump`` (and its  derivatives for the different platforms like ``aarch64-linux-gnu-objdump``).  Take this line as an example:   ``[  +0.000240]  rkvdec_device_run+0x50/0x138 [rockchip_vdec]``.   We can find the corresponding line of code by executing::     aarch64-linux-gnu-objdump -dS drivers/staging/media/rkvdec/rockchip-vdec.ko | grep rkvdec_device_run\>: -A 40    0000000000000ac8 <rkvdec_device_run>:     ac8:	d503201f 	nop     acc:	d503201f 	nop    {     ad0:	d503233f 	paciasp     ad4:	a9bd7bfd 	stp	x29, x30, [sp, #-48]!     ad8:	910003fd 	mov	x29, sp     adc:	a90153f3 	stp	x19, x20, [sp, #16]     ae0:	a9025bf5 	stp	x21, x22, [sp, #32]        const struct rkvdec_coded_fmt_desc *desc = ctx->coded_fmt_desc;     ae4:	f9411814 	ldr	x20, [x0, #560]        struct rkvdec_dev *rkvdec = ctx->dev;     ae8:	f9418015 	ldr	x21, [x0, #768]        if (WARN_ON(!desc))     aec:	b4000654 	cbz	x20, bb4 <rkvdec_device_run+0xec>        ret = pm_runtime_resume_and_get(rkvdec->dev);     af0:	f943d2b6 	ldr	x22, [x21, #1952]        ret = __pm_runtime_resume(dev, RPM_GET_PUT);     af4:	aa0003f3 	mov	x19, x0     af8:	52800081 	mov	w1, #0x4                   	// #4     afc:	aa1603e0 	mov	x0, x22     b00:	94000000 	bl	0 <__pm_runtime_resume>        if (ret < 0) {     b04:	37f80340 	tbnz	w0, #31, b6c <rkvdec_device_run+0xa4>        dev_warn(rkvdec->dev, "Not good\n");     b08:	f943d2a0 	ldr	x0, [x21, #1952]     b0c:	90000001 	adrp	x1, 0 <rkvdec_try_ctrl-0x8>     b10:	91000021 	add	x1, x1, #0x0     b14:	94000000 	bl	0 <_dev_warn>        *bad = 1;     b18:	d2800001 	mov	x1, #0x0                   	// #0     ...   Meaning, in this line from the crash dump::     [  +0.000240]  rkvdec_device_run+0x50/0x138 [rockchip_vdec]   I can take the ``0x50`` as offset, which I have to add to the base address  of the corresponding function, which I find in this line::     0000000000000ac8 <rkvdec_device_run>:   The result of ``0xac8 + 0x50 = 0xb18``  And when I search for that address within the function I get the  following line::     *bad = 1;    b18:      d2800001        mov     x1, #0x0 **Copyright** ©2024 : Collabora